Amplifying system



July 17, 1934. A. D. SILVA AMPLIFYING SYSTEM Original Filed July 25, 1928 lwli i mm a'mi Patented July 17, 1934 UNITED STATES FATE FFECE AMPLIFYING SYSTEM Application July 25, 1928, Serial No. 295,200

Renewed May 3, 1933 8 Claims.

My invention relates to thermionic amplifiers and their associated circuits utilizable for amplifying fluctuating currents or oscillations, and more particularly currents of the character prescut in radio receiving apparatus.

In accordance with my invention regeneration caused by the capacity coupling between the grid and anode circuits of a thermionic device, particularly by the grid-plate capacity, and particularly when the regeneration is of a magnitude to cause or tend to cause production of local oscillations, is more or less completely suppressed or counteracted, either to a degree to prevent production of local oscillations, or to a greater degree which will more or less completely counteract or reduce the effect of regeneration Well below the point of self-oscillation.

In accordance with my invention, for the purposes aforesaid, there is utilized, solely or in association with other stabilizing or neutralizing means, in either the input or output system of the thermionic amplifier, utilized to amplify either radio or audio frequency current, a damping resistance in series with a small condenser or capacity, external to or other than the amplifier tube or an internal inter-electrode capacity thereof, connected across or between a pair of electrodes of the amplifier tube; and in this relation and for this purpose the series capacity or condenser is of such small magnitude that its reactance throughout the range of frequencies involved is large as compared to the resistance in series therewith.

More particularly in accordance with my invention the series combination of resistance and capacity is of advantage in connection with tuned radio frequency amplifiers in which the input and output systems of an amplifier tube each includes a loop circuit tunable substantially solely by variable capacity throughout a wide range of frequencies. In such a system the output system has the characteristic that the tendency of the amplifier system to become unstable increases throughout the wave length range as the input and output systems are tuned to the same shorter and shorter wave lengths of that range. The connection of the series combination of resistance and small capacity external to the tuned loop circuit of either the input or output system is markedly effective in reducing the tendency to decreasing stability with increasing frequency, for under the conditions of fre-- quency change the effect of the resistance with its series capacity very rapidly increases, as the system is tuned to shorter and shorter wave lengths.

Further in accordance with my invention there is utilizable, preferably in series with the aforesaid combination of resistance and capacity, a combination of another resistance and capacity in parallel with each other.

My invention resides in a system of the character hereinafter described and claimed.

This application is in part a continuation of my prior application Serial No. 727,866, filed July 24, 1924.

For an understanding of my invention and for an illustration of some of the various forms it may take, reference is to be had to the accompanying drawing, in which:

Fig. 1 is a diagrammatic illustration of a radio receiving circuit embodying my invention.

Fig. 2 is a diagrammatic view of a radio receiving system embodying a modification of my invention.

Referring to Fig. 1, D is an antenna or structure for absorbing electro-radiant energy directly from the natural media or generically any circuit or system of transmission by wire between stations of a fluctuating or alternating current of any frequency, including carrier wave and radio frequencies.

Between the antenna D and earth or other counter-capacity E is connected the primary P of a preferably step-up radio frequency transformer '1 across whose secondary S is connected the variable tuning condenser C, S and C forming, in the input system of the first radio frequency amplifier tube V, a loop circuit tunable by fixed inductance, S, and variable capacity C.

Directly, or in any event eliectively, across the terminals of the grid 9 and filament or equivalent cathode f of the tube V, is connected an elernent comprising, in series with each other, the damping or stabilizing resistance R and condenser or capacity K, which is external to the tube V and distinct from, but in some instances co-operatively related to, intra tube capacity or capacities, or capacity or capacities external to the tube and existing between its electrodes, as formed by socket terminals, electrical connections, etc. In the anode or plate circuit of the tube V, and having its one terminal connected to the anode A, is the primary P1 of a step-up radio frequency coupling transformer T across whose secondary S1 is connected the tuning condenser Cl, S1 and Cl again forming a closed loop circuit whose inductance is fixed and is tunable by the condenser C1, and comprised in the output system of the tube V and more directly in the input or grid circuit of the second radio frequency amplifier tube V1. Similarly related to the electrodes of the tube V1 and to the tuned loop S1, C1, as in the case of the preceding tube, is a second element KR. In the output or plate circuit of the tube V1 is the primary P2 of a step-up radio frequency coupling transformer T2 whose second" ary S2 is bridged by the tuning condenser 02. This tuned loop is in the output system of the tube V1 and more directly in the input or grid circuit of the detector tube V2, in whose anode circuit is I which represents a telephone, signal translating instrument of any type, or an audio frequency transformer effecting coupling to an audio frequency amplifier which may be the first of several in cascade. In its grid lead is the usual condenser is shunted by grid leak r.

In the circuits herein illustrated the currents for the filaments j are derived from a battery A or from any suitable alternating source; and the current for the anode circuits is obtainable from a battery B, or from any filter-rectifier system supplied from a source of alternating current. The currents for the filaments 1 may be controlled by individual rheostats, or by rheostat common to all the filaments, as well understood in the art.

In the grid circuit of the tube V2 there may again be utilized a stabilizing element KR though it may often be omitted from the input of the detector tube though it may be considered as having some stabilizing effect by introducing a damping resistance, whose effect varies with frequency, in the plate circuit of the preceding tube V1. It also automatically increases the resistance of the grid circuit of the detector tube V2 as it is tuned to higher and higher frequency, with desirable effect upon the plate circuit of tube Vl. Similarly the element KR in the grid circuit of the tube V1 may be considered as having some damping effect variable with the frequency, in the plate circuit of the tube V.

The significant effect, however, of the element KR is in the sense or way of automatically increasing the positive resistance of the tuned grid circuit of a tube whose output circuit or system is similarly tunable throughout a wave length range and tending to cause regenerative feed back or introduction of negative resistance into the tuned grid circuit at a rate which increases rapidly with increase in frequency.

The effect of the element KR increases practically as the fourth power of the frequency, and hence its effect increases very rapidly with increase of frequency to which the input and output systems are simultaneously tuned.

Whatever the magnitude of the condenser K of each stabilizing element, the resistance R becomes more effective with increase in frequency. B may have resistance values ranging from about 100 ohms to about 25000 to 30000'ohms, depend ing upon the-frequency range, amount of regeneration or magnitude of tendency to oscillate, and

magnitude of associated capacity K, and upon other considerations. v

Condenser K preferably has a capacity such that its reactance at agiven frequency is larger than and preferably great as compared to the resistance of R. For a wave length range of from about 200 to about 600 meters to which the input and output systems are tunable, the reactance of K atabout 600 meters and about 200 meters may be aboutsix times and about twice, respectively, the resistance of R.

For broadcast receivers tunable for example throughout a Wave length range of from about 200 to about 600 meters, the resistance B may have a magnitude ranging from say about 100 or 200 ohms to about 1,000 or 1,500 ohms, depending upon conditions of the character above referred to; and the capacity of the condenser K may be of the order of an interelectrode capacity of the tube, and may range, for example, from about 2 or 3 to or micromicrofarads.

It is desirable also that the magnitude of capacity Kshall not be large enough, compared with the minimum capacity of the associated tuning condenser and other small capacities in parallel therewith, materially to affect the tuning range.

The minimum capacity of a tuning condenser such as for example C1, is ordinarily of the order of 20 micromicrofarads, in parallel with which are the grid-filament capacity, about 3 to 8 micromicrofarads, of the tube V1, and 2 or 3 micromicrofarads occasioned by the wiring, socket, etc. Capacity K may be of the order of magnitude of the sum of these capacities, and is Re. ct. e f K K (m. m. r. g lg f R 111 ohms) i 2. 5 I 42, 400 ohms 16,000 5. 21, 200 4, 000 10. 10,600 1 000 20. 5, 300 250 Referring to Fig. 2 the arrangement is much the same as that of Fig. 1 except that in series with each element BK, is another element comprising a resistance R1 in parallel with a condenser K1; and reversed poling of the coupling transformers is not shown, though it may be used also in this circuit arrangement. I

The element RlKl is the equivalent of acondenser having substantial or high dielectric loss, and such a condenser may be substituted for element RlKl.

With increasing frequency the effect of the damping or stabilizing effect of the resistance R1 decreases, and by a combination of the two types of elements, one the series type, the other the parallel type, the latter or parallel type may reduce, govern or control the rate of increase of effect of resistance with increase in frequency.

In the reduction or elimination of regeneration or, regenerative effects the stabilizing effects of the elements hereinbefore described may be supplementedby systems for neutralizing or balancing the effect of the grid plate capacity, which systems, however, need not effect complete; neutralization or balancing of the capacity between the grid and plate circuits since the stareversed so that at all instants the grid terminal of the secondary winding and the plate terminal of the primary winding are at opposite alternating current polarities or potentials. This reverse poling of the coupling transformer is effective through any natural or inherent capacity existing between the grids of the tubes immediately preceding and following the transformer, partially or more or less completely to balance or neutralize the grid-plate feed-back capacity of the tube in advance of that transformer.

In Fig. 2, however, the reverse poling of coupling transformers is omitted, though it or equivalent means may be resorted to also with this circuit arrangement. When the reverse poling or equivalent is omitted there is much greater tendency to instability, and the resistance R, whether or not element RlKl be present, will be of materially greater magnitude. For example, in Fig. 2, the tubes when of the WD-ll type, having a grid-plate capacity of about 3 micromicrofarads, and each resistance R may be of the order of 25000 or-30000 ohms, for 600 meter wave length or for a wave length range of 400 to 800 meters. And with grid-plate capacities of greater magnitude these magnitudes of resistances B may be suitable to stabilize, especially when reverse poling or equivalent effect is absent.

The step-up ratios of the coupling transformers T1, T2, will generally be of the order of ten for enhancing selectivity and because of the few primary turns limiting amplification or maintaining suitably low the inductive reactance of the anode circuit, and in that way maintaining low the regenerative action or tendency thereto.

While the resistance R, or R combined with R1, has a damping effect upon the related tuned loop or loops, that damping efiect is such as not materially or seriously to affect the sharpness of resonance or tuning obtained, and since the damping eifect is greater at high frequencies, the eifect of the resistance or resistances upon sharpness of tuning decreases with decrease in frequency.

The tubes V, V1 above referred to may be of any suitable type, such as that known as 201A or the corresponding present tubes whose filaments are heated by alternating current. It will be understood, however, that my invention is not limited to tubes of these particular characteristics, but is of general application for wave length ranges other than that above indicated and for tubes having characteristics other than those indicated.

What I claim is:

1. A system of the character described comprising an electronic tube, input and output circuits therefor, a fixed resistance connected between a pair of the terminals of said tube, a fixed condenser in series with said resistance in the connection thereof, and an element including a resistance and condenser in parallel with each other and in series with such first named resistance and condenser in said connection.

2. The combination with a thermionic amplifier tube, of input and output systems therefor, each of said systems including a closed loop circuit having fixed inductance and tunable throughout a wave length range substantially solely by a variable tuning condenser in shunt with said inductance, and a stabilizing element, external to the tunable loops of both said input and output systems and directly associated with one of them, comprising a capacity external to said tube in series with a resistance whose magnitude is small compared to the magnitude of the reactance of said capacity throughout said wave length range, and a second element for modifying the change of effect of said stabilizing element with change of frequency comprising a resistance and a capacity in parallel with each other and in series with said stabilizing element.

3. The combination with a thermionic amplifier tube, of tunable input and output systems therefor, means for partially balancing the grid-plate capacity of said tube, and means complementary to said balancing means for maintaining stability throughout the range for which said systems are tunable comprising a capacity external to said tube in series with a resistance whose magnitude is small throughout said wavelength range, and a combination of resistance and capacity in parallel with each other and in series with said first resistance and capacity.

4. lhe combination with a thermionic amplifier tube, of input and output systems therefor, each of said systems including a closed loop circuit having fixed inductance and tunable throughout a wave length range substantially solely by a variable tuning condenser in shunt with said inductance, and a stabilizing element external to the tunable loops of both said input and output systems and directly associated with one of them, comprising a capacity in series with a resistance whose magnitude is small compared to the magnitude of the reactance of said capacity throughout said wavelength range, and a second element for modifying the effect of said stabilizing element with change of frequency comprising a combination of resistance and capacity included in one of said systems external to the tunable loop thereof.

5. A system of the character described comprising an electronic tube, input and output circuits therefor at least one of which is tunable through a range of frequencies, a fixed resistor connect d between a pair of the terminals of said tube, a reactor in series therewith between said terminals whose reactance throughout said range of frequencies is substantially greater than the resistance of said resistor, and an element comprising resistance and reactance in parallel to each other and in series with said first-named resistance and reactance between said pair of terminals.

6. The combination with a thermionic amplifier tube, of tunable input and output systems therefor, means for partially balancing the gridplate capacity of the tube, and means complementary to said balancing means to maintain stability throughout the range for which said systems are tunable comprising a resistor in series with a reactor whose rcactance throughout said range of frequencies is substantially greater than the resistance of said resistor, and a combination of reactance and resistance in parallel with each other and in series with said first resistance and reactance.

ALBERT D. SILVA. 

